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Conveying Bends Article Paul Solt

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Pneumatic conveying of bulk solids has been successfully practiced in industries as diverse as chemical, agricul-tural, pharmaceutical, plastics, food, mineral processing, cement and power generation for more than a century. Pneumatic conveying provides advan-tages over mechanical conveying sys-tems in many applications, including those that require complex routing, multiple source-destination combina-tions and product containment. Pneumatic conveying transfer lines are often routed over pipe racks and around large process equipment, giving process operators great layout flexibility. Such de-sign flexibility is made possible by the use of bends (such as elbows and sweeps, dis-cussed below) between straight sections (both horizontal or vertical), which enable convenient change of direction in the flow of the conveyed solids. However, among all the components of a pneumatic conveying system, bends despite their apparent simplicity are probably the least understood and most potentially problematic for process op-erators. Findings from various research studies are often not consistent, and often times public findings do not match field experience. The importance of bends in any pneu-matic conveying assembly cannot be over-stated since if not properly selected and designed they can contribute sig-nificantly to overall pressure drop, prod-uct attrition (degradation) and system maintenance (due to erosive wear). Historically, a basic long-radius bend (shown in Figures 1 and 2, and discussed below) has been the bend of choice for de-signers of pneumatic conveying systems, for a variety of reasons: Long-radius bends provide the mostgradual change in direction for solids, and hence are most similar to a straight section of piping Theangleofimpactonthepipewallisrelatively small, which helps to mini-mize the risk of attrition or erosion For lack of other experience, to main-tain the status quoYears of field experience and a variety of studies conducted to troubleshoot com-mon problems such as line plugging, excessive product attrition (degradation), unacceptably high bend wear and higher-than-expected pressure drop clearly indicate that the flow through bends in pneumatic piping is very complex. One should refrain from generalizing the find-ings until the underlying physics are well understood.This complexity is exacerbated when innovative designs are introduced to ad-dress existing issues with common-radius bends (also discussed below). Today, most of the data still resides with vendors and there is a need for fair, unbiased and tech-nically sound comparative evaluation. The purpose of this article is to summa-rize the key concepts, outline key metrics used to evaluate bend performance, and provide guidance for their selection. We will limit our discussion to dilute-phase conveying. (Issues related to pipe bends for dense-phase conveying systems will be addressed at a future date.)BackgroundBends are installed in a pneumatic con-veying system wherever a change in di-rection is required along the conveying route. They can be broadly classified into three major categories:a. Common-radius bends (including el-bows, short-radius, long-radius and long-sweep bends)b. Common fittings (including tee bends, mitered bends and elbows)c. Specialized bends and innovative de-signs (such as the Gamma bend, Ham-mertek Smart Elbow, Pellbow, wear-back designs, and lined bends, which are described in the next section)a. Common-radius bends. Common-radius bends (as shown in Figures 1 and 2) are made by bending standard tubes or pipes. The radius of curvature (RB) may range from 1D to 24D (where D is the diameter of the tube or pipe). Common-radius bends can be loosely classified as follows:Elbow: RB/D = 1 to 2.5Short radius: RB/D = 3 to 7Longradius: RB/D = 8 to 14Longsweep: RB/D = 15 to 24Solids Processing53 ChemiCal engineering www.Che.Com april 2009Solids ProcessingImpact / wearzonesReaccelerationzoneDRB+ Ricocheting PatternSliding PatternD = Pipe diameter, RB = Bend radius RB/ D = 8 to 14Understanding Bends In Pneumatic Conveying SystemsFigure 1. Flow in a standard, long-radius bend is illustrated here, with typi-cal fow patterns, wear points and reacceleration zone shownDespite their apparent simplicity, bends are often poorly understood and unless properly designed, they are potentially problematicImpact / wearzonesRB+ReaccelerationzoneDRicocheting PatternLEGEND FOR FIGURES 1 AND 2Sliding PatternD = Pipe diameterRB = Bend radius RB/D = 3 to 7Figure 2. Flow in a standard, short-radius bend is illustrated here, with typical fow patterns, wear points and reacceleration zone shownShrikant DhodapkarThe Dow Chemical Co.Paul SoltPneumatic Conveying ConsultantsGeorge KlinzingUniversity of PttsburghThese bends are available in wide range of materials of construction and thicknesses, similar to the straight sec-tion of pipe (tangent) that is provided on either side of the curved section. The conveyed material may undergo multiple impacts with the pipe wall, or may slide along the outer radius, depending on ma-terial properties, solids loading and gas velocity. Bend wear and material attrition comonly occur at the impact zones.b. Common fittings. The most com-monly used fitting to accomplish a change in flow direction is a blind tee bend. In this design, one of the outlets is plugged thereby allowing conveyed solids to accu-mulate in the pocket (Figure 3). The ben-efit of this design is that the accumulated pocket of material cushions the impact of the incoming material, significantly re-ducing the potential for wear and product attrition. The extent of accumulation in the pocket will depend on the orientation of the bend, solids loading, gas velocity and material properties (such as particle size and cohesiveness).However, in a tee bend, the conveyed solids lose most of their momentum dur-ing the impact and thus must be reacceler-ated downstream of the bend. As a result, pressure drop across a blind tee can be as much as three times that of a long-radius bend. Several variations of tee bends are shown in Figure 4.c. Specialized bends. Today, a variety of specialized designs are available to control flow within the bend, in order to minimize attrition and wear. This is often achieved by creating a self-cleaning or replenishing pocket or layer of material, upon which the incoming stream impinges. Wear in-side the piping is minimized by redirect-ing the gas-solid suspension away from typical wear points. Several of the most commonly used specialized bends are dis-cussed in the following section.Gamma Bend. The Gamma Bend from Coperion (coperion.com) is shown in Figure 5. Its innovative design relies on creating particle-particle impact in the impact zone and prevents sliding motion of particles along the outer ra-dius to minimize particle smearing, so it is especially effective in preventing the formation of streamers or angel hair in polymer pellets. A minimum solids loading of 5 (defined as mass of solids/mass of air) which depends on the bulk density of the product, is required to ensure accumulation of material in the impact zone. In the absence of this layer, the particles will directly impact the target plate within the bend and may result in both par-ticle attrition and pipe erosion. These bends are typically fabricated from stainless steel, and provide a very tight bend radius (RB/D = 4 to 6). The pressure drop is higher (20-40%) than that experienced by a typical short-radius bend (RB/D = 3 to 7). Pellbow Bend. The Pellbow Bend from Pelletron Corp. (pelletroncorp.com) is shown in Figure 6. It is similar to a short-radius bend but has an ex-panded pocket. The pocket is meant to accumulate a small amount of solids at the primary impact location so that most of the impact is between particles themselves. To ensure adequate accu-mulation of material in this pocket, the minimum recommended solids loading is 3 (mass of solids/mass of air). According to the vendor, pressure drop will be slight higher than that ex-perienced by a short-radius bend. It is available in wide range of materials of construction. Vortice-Ell Smart Elbow or Ham-mertek - Smart Elbow. The Vor-tice-Ell Smart Elbow from Rotaval (rotaval.co.uk) and the Hammertek Smart Elbow from Hammertek Corp. (hammertek.com), are similar in design (Figure 7). Both have a bul-bous extension on the heel. Depending on the orientation and inlet gas veloc-ity, the incoming material will either fill the chamber or circulate within the chamber before exiting. In either case, it results in significant reduction in wear and attrition of material. It is available in 45- and 90-degree designs and in various materials of construc-tion.Wearback designs. There are two major types of wearback elbow de-signs (as shown in Figure 8):a. Equipped with a wear plate with a sacrificial and replaceable back plate: Thereplaceablebackplateismadefrom hardened material, typically with Brinell hardness greater than 400 (e.g., Ni hard) Typicallyavailableinshort-radiusdesigns (RB/D = 2 to 6) and mul-tiple angles 22.5, 45, 60 and 90 de-grees Segmented designs are available,which allows for partial replace-ment of the elbow body Commonly used in the flyash in-dustryb. Tube-in-tube (pipe-in-pipe) arrange-ments: The space between the inner andouter casings can be left unfilled or filled with concrete or porcelain or another abrasion-resistant ma-ter